The present invention relates to a power supply circuit, a display driver, an electro-optical device, and an electronic instrument.
A further reduction in power consumption is demanded for portable electronic instruments. A liquid crystal device is generally used as a display device provided in such electronic instruments. A plurality of power supply voltages (e.g. high voltage and negative voltage) are required to drive the liquid crystal device. In this case, it is desirable from the viewpoint of cost that a liquid crystal driver device which drives the liquid crystal device include a power supply circuit which generates power supply voltages.
Such a power supply circuit includes a voltage booster circuit. A charge-pump circuit which generates a voltage boosted by a charge-pump operation is generally employed as the voltage booster circuit. The charge-pump circuit connects one end of a capacitor storing a charge with various voltages using switching elements (e.g. metal oxide semiconductor (MOS) transistors), thereby boosting a voltage corresponding to the charge stored in the capacitor. Power consumption can be reduced using such a charge-pump circuit.
In order to increase the boost efficiency of the charge-pump circuit, the loss of charge must be reduced as much as possible. Therefore, the charge-pump circuit is configured so that the switching elements have an on-resistance as low as possible. However, since a charge is not stored in the capacitor immediately before activating the charge-pump circuit, a large current (rush current) flows immediately after startup.
In order to solve this problem, a boost operation is performed using a soft-start circuit immediately after startup. Such a soft-start circuit is disclosed in JP-A-2005-57860, for example.
When using a charge-pump circuit as a first-order voltage booster circuit, a second-order voltage booster circuit, a third-order voltage booster circuit, a regulator, or the like (power supply generation circuit in a broad sense) operates using a boost voltage generated by the first-order voltage booster circuit as a power supply voltage. The second-order voltage booster circuit or the like is generally started up after the startup timing of the first-order voltage booster circuit. This is because the second-order voltage booster circuit or the like aims at stably utilizing the boost voltage of the first-order voltage booster circuit or preventing latchup caused by a large current which occurs immediately after startup of the second-order voltage booster circuit or the like.
On the other hand, the second-order voltage booster circuit or the like is also connected with a flying capacitor for a charge-pump operation and an output stabilization capacitor. Therefore, the second-order voltage booster circuit or the like is configured so that a switching element (transistor) in the power supply output stage has a low on-resistance in order to enable efficient charge storage, for example. In this case, the technology disclosed in JP-A-2005-57860 can prevent a rush current when activating (turning ON) the first-order voltage booster circuit, but cannot prevent a rush current when activating the second-order voltage booster circuit or the like.